This invention generally relates to frames for holographic elements, and more specifically to a frame particularly designed to facilitate locating holographic recording mediums, holographic lenses and matched filters precisely in position in systems such as holographic recording and correlator systems.
Matched filters are made by exposing holographic recording mediums to certain types of diffraction patterns, and then developing the exposed recording mediums to produce holograms. These filters are used in optical correlator systems by directing certain light beams through the holograms on the matched filters and then processing the output therefrom to detect the presence or absence of a selected target in a scene or field of view. Because very little space is required on a matched filter to store a hologram, it is possible to construct a filter having a large array of holograms, and, for example, a 50 mm.times.50 mm matched filter may be made with a 100.times.100 array of holograms.
Such a matched filter may be fabricated using a multiple beam generating holographic lens to generate an array of beams, and directing each of these beams toward a different area on a recording medium to produce an array of diffraction patterns thereon. Such a filter may be employed in an optical correlator system by using a similar or identical holographic lens to generate an array of beams, and directing each of these beams to a different hologram on the matched filter.
To operate the correlator system effectively, it is very important that the matched filter be located, with a very high degree of precision, in a specific position relative to the multiple beam generating holographic lense. Not only is the distance between these two elements critical, but their relative angular orientation and their relative lateral placement are also very important. For instance, the correlator system may be ineffective if the matched filter is laterally placed only 50 to 100 microns away from where it should be relative to the multiple beam generating lense.
The correct relative position of the matched filter and the multiple beam generating lens in the correlator system depends on the position of the recording medium, from which the filter was made, relative to the multiple beam generating lens used to form the array of diffraction patterns on that recording medium. Because of this, in order to use the matched filter properly in the correlator system, it is also highly desirable that the recording medium, from which that filter was made, be located very accurately relative to the multiple beam lens used in the construction of the matched filter.
Holographic systems are known that, with a relatively few adjustments in the components of the system, may be operated both to make a matched filter on a photographic plate, and also as an optical correlator using that filter, and where the correct position of the matched filter in the system, when it is used as a correlator, is identical to the position of the photographic plate form which that matched filter was made. Such systems substantially facilitate determining the correct position of the matched filter; however even with these systems, it is still very important to precisely position the photographic plate in the system, remove it, develop it, and then precisely position the developed matched filter back in the system.
Achieving this very high degree of precision in holographic recording and optical correlator systems is very difficult and time consuming for several reasons. For instance, in these systems, the necessary placement and adjustment of the recording medium, the matched filter, and the mulitple beam generating holographic lens are normally done either in drak or in very low levels of light to minimize any undesirable exposure of the recording medium or the matched filter. Moreover, holographic recording mediums and matched filters usually include glass plate backings that often have rough outside edges, and these rough edges may cause the plates to tilt slightly if they are seated against edges of conventional frames or flat support surfaces. WHile this tilt may be very slight, it may produce a very undesirable deviation in the position of a holographic element. Also, sliding a rough edge of a glass plate against a flat or round pin support surface may cause the whole plate to chip along the edge or to crack.